Precious few garments have been made of spider silk. In 2012, a cape and shawl made from natural spider silk were displayed at the Victoria and Albert Museum, where visitors learned that the garments were the result of a unique project that spanned eight years and involved the harvesting of silk from 1.2 million spiders. In 2019, a rather less painstaking project utilized fibroin, the protein found in natural spider silk, to fabricate an outerwear jacket, North Face’s Moon Parka. Starting with fibroin meant that silk could be sourced from genetically modified bacteria, which are easier to work with than spiders. Nonetheless, the Moon Parka, which takes its name from the word moonshot, was never meant to be mass produced. It was available by lottery for just a limited time.
Museum pieces and moonshots are hardly synonymous with “mass production.” Is there another way to generate spider silk–based textiles, one that has more commercial potential? Yes, according to Kraig Biocraft Laboratories, which uses transgenic silkworms to produce lines of recombinant spider silk. The company plans to produce up to 10 metric tons of spider silk in 2025. Production of actual spider silk lines on this scale would allow textile manufacturers to test the silk on their own equipment.
It’s not just textiles that may benefit. Recombinant spider silk’s tensile strength, weight, and durability make it attractive for myriad applications, including tissue scaffolds and sutures in the biomedical field, as well as textiles and ballistic materials.
Modified silkworms
“In a silkworm, there are several proteins that are produced in the silk glands,” Kim Thompson, founder and CEO of Kraig Biocraft Laboratories, tells GEN. “One of those—a heavy chain fibroin—contributes roughly 96–98% of the molecular weight of the fiber.” Replacing the gene responsible for producing that protein with its counterpart in the spider results in recombinant spider silk.
Kraig Biocraft produces spider silk using hybrid silkworms. As Thompson explains, hybrids of the two parental strains are more vigorous and produce better shaped cocoons. That vigor is passed down to subsequent generations.
Kraig Biocraft’s approach appears to be unique in the spider silk industry. Other companies use vat fermentation to produce proteins that must be extracted, purified, and transformed into threads, adding steps and costs to the overall process.
Diverse applications
Spider silk’s high strength and light weight have attracted the interest of the U.S. Department of Defense. Dragline spider silk (which spiders use for the radial lines of their webs) requires 120,000–160,000 J/kg to break, whereas Kevlar requires 30,000–50,000 J/kg and steel requires 2,000–6,000 J/kg. Dragline spider silk weighs 1.18 and 1.36 g/cm3, whereas Kevlar weighs 1.44 and steel weighs 7.84. Because spider silk combines strength, biocompatibility, and elasticity, it could be useful in tissue matrices and sutures. Dragline silk can increase its length by 27%, and flag silk (which spiders use for the spiral lines of their webs) can increase its length by 270%.
Spider silk—or rather the technology behind it—could also be of interest to biopharmaceutical companies. For example, transgenic silkworms could serve as expression and production platforms for proteins other than spider silk proteins. Still, for Kraig Biocraft, the most immediate applications are in materials science. “We haven’t branched out into other areas that require more regulatory approval yet,” Thompson says.
Rethinking the problem
Thompson first approached the challenge of producing spider silk about 20 years ago. “I was looking at all the companies involved in that space,” he recalls. “The leader, Nexia Biotechnologies, was producing spider silk proteins in the milk of dairy goats.”
“I thought that Nexia had misdiagnosed the problem and that it was about to hit a wall,” Thomson continues. Nexia’s method not only had difficulty with the mechanical challenges of transforming the proteins into fibers, but it was also extremely expensive.
Thompson thought it would be better to create a cohesive fiber with the desired mechanical characteristics, than to create spider silk protein. He even suggested to Nexia that its scientists should use genetically engineered silkworms to produce fibers rather than using dairy goats to produce proteins. Nexia, however, preferred its approach, which yielded small proteins that were too weak to be spun into fiber. It declared bankruptcy in 2009.
The University of Wyoming (UW), which held the rights to the genetic sequences Nexia has used to produce spider silk protein, granted Thompson exclusive rights to those sequences. “UW’s chief scientist, who had worked with Nexia, listed five reasons why it was scientifically impossible for these sequences to work in silkworms,” Thompson says.
What that scientist may not have considered was that Thompson, working with molecular geneticist Malcolm J. Fraser, PhD, who then headed a laboratory at the University of Notre Dame, had a way to insert those sequences into silkworms. Fraser had co-developed the piggyback transposon, which “at the time was the only way to genetically engineer silkworms,” Thompson notes.
Nonetheless, objections raised by the UW scientist reemerged every time Thompson approached a venture capital company for financing. Only by demonstrating dogged persistence did Thompson finally secure Kraig Biocraft the funding it needed to develop spider silk suitable for use by textile mills. Today, the company looks forward to starting commercial-scale production.
Current challenges
“Our next inflection point is to produce the first metric ton of spider silk,” Thompson says. He adds that he is in discussions with “a number of significant players” to test Kraig’s recombinant spider silk on their machinery. The limiting factor, until now, has been an insufficient supply of product. “It’s hard to run a test when the world supply of spider silk has been measured in tens of kilograms,” he points out.
To overcome supply problems, Kraig Biocraft plans to make good use of its new manufacturing site. “We have a backlog of order for prototype materials so they can make a test run,” Thompson says. According to Kraig Biocraft’s website, a kilogram of recombinant spider silk costs less than $300 to produce—about one tenth the cost of the protein alone using vat fermentation production methods.
“To my knowledge, there are only three other companies involved in making spider silk: AMSilk, Bolt Threads, and Spiber,” Thompson says. Each uses vat fermentation to make the spider silk proteins, which he says significantly increases the costs.
Thompson envisions a future of composite fibers in which spider silk is mixed with other textiles: “A lot of work continues to be done in that area, and it is accelerating.” He also points out that there are thousands of markets and technical applications for the advanced materials that are possible using recombinant spider silk: “We’re interested in capturing as much market share as we can, and we’re looking at new and expanded mechanical properties.”
In the very near future, recombinant spider silk may be found in a range of products, from tissue scaffolds and sutures to performance fabrics. In that world, capes or expedition jackets made of spider silk won’t be rarities. They’ll be off-the-shelf articles.
